Previously, it is known that cranking of an internal combustion engine, which is executed after execution of idling reduction (also referred to as idling stop or no idling) of the internal combustion engine, causes a drop of a power source voltage, which is supplied to an electric power steering (EPS) apparatus. JP2013-163515A (corresponding to US2011/0231064A1) discloses an electric power steering apparatus, which maintains steering assistance for assisting a steering operation of a driver of a vehicle while limiting the electric current supplied to the motor at the time of dropping the electric source voltage.
However, depending on the time of cranking, the electric source voltage may possibly drop below a required minimum operating voltage of an electronic control unit (EPS-ECU) of the electric power steering apparatus to possibly cause rebooting of the EPS-ECU or stopping of the operation of the EPS-ECU. In such a case, the electric power steering apparatus cannot maintain the steering assistance, so that the driver of the vehicle may have annoying feeling.
In view of the above point, it is conceivable to connect a DC-DC converter between the battery and the EPS-ECU to input a voltage, which is boosted by the DC-DC converter, to the EPS-ECU. The EPS-ECU includes an electric power converter, which drives the motor, and a controller, which controls an operation of the electric power converter. A required electric power of the electric power converter and a required electric power of the controller are different from each other. Therefore, when the DC-DC converter is designed in conformity with the electric power converter, which requires the relatively high voltage, in order to limit the drop of the input voltage, which is inputted to the EPS-ECU, the size of the DC-DC converter is unduly increased.